The alignment of a vehicle's wheel plane relative to the path traveled by the vehicle affects not only the handling of the vehicle but also affects the wear on the tires. As used here, alignment refers to camber, toe, and thrust. Camber is the angle between the vertical axis of the wheel and the vertical axis of the vehicle. Positive camber refers to an angle where the top of the wheel is farther away from the center of vehicle than the bottom of the wheel. Negative camber refers to an angle where the bottom of the wheel is farther away from center of the vehicle than the top. Generally speaking, camber changes of even a fourth of one degree can impact tire wear. Abnormal tire wear has been observed in certain applications with even smaller camber angles changes. Toe is the angle each wheel makes with the longitudinal axis of the vehicle. Positive toe, also referred to as toe in, is a condition where the front of the wheel is pointing in or towards the center line of the vehicle. Negative toe, also referred to as toe out, is a condition where the front of the wheel points out or away from the center line of the vehicle. Thrust is the resulting direction of travel of an axle as opposed to the direction that might be expected from the orientation of the tires on the axle. Generally speaking, toe changes of even one-tenth of a degree can have an impact on tire wear.
The typical trailer axle is made by welding a pair of spindle forgings onto a piece of axle tubing then machining the precision surfaces of both spindles simultaneously in a lathe process. The resulting axle is near perfectly straight; i.e., each spindle axis possesses zero camber and zero toe. When a typical axle is installed under a vehicle (used herein to refer to both motorized vehicles as well as trailers) and placed into normal operation under typical loading conditions, the camber does not remain at zero. The axle under load, although quite rigid, does flex. The flexing of the axle occurs because the suspension is attached to the axle at load transfer points which are significantly inboard of the ends of the axle, but the tires support the weight of the vehicle by means of attachment points which are relatively near the outboard ends of the axle. As a result of this geometry, the weight of the vehicle imposes a bending moment on the axle which in turn causes upward deflection of the ends of the axle resulting in the tires presenting a slight negative camber. As the load increases, the more negative the camber becomes. At the typical maximum legal tandem axle load in the US, it would not be unusual for the wheel camber angle to reach approximately 0.5 degrees. The contribution of tire alignment to tire wear can be particularly problematic with vehicles used for transporting heavy loads.
Once the weight is removed, the axle may recover and again affect the alignment of the wheels. Because of factors such as the additional costs and amount of material that would be required, increasing the stiffness of the axle to resolve camber issues may not be practical.
Even with the same amount of camber on each axle spindle, axle camber affects the tires differently depending on their individual wheel end position on the vehicle because most road surfaces are not flat transversely across the road. The road surface is either crowned or sloped (by about 1.5% on average) so that water will evacuate from the road surface. Trucks, in most of the world, generally operate in the right most lane, and the right most lane is usually sloped very slightly to the right. This means that all the while the vehicle is traveling on the road way, there is a gravitational pull on the rig that is pulling the vehicle to the right. This pull is resisted through the tire contact patch and the tire transmits this force to the axle by transmitting the required force opposite of the direction of pull through its interface with its wheel. The result is that as the tire rolls down the highway, the contact patch shifts leftward with respect to the wheel center. At full load and at normal pressure on a typical NGWBS tire, this shift has an effect on tire shoulder wear that is roughly the equivalent of a 0.5 degree shift in wheel camber. This means that, although the left and the right wheel may each measure approximately −0.5 degree of camber, when the shift effect is considered, the effective camber angle on the left side tires is approximately −0.7 degree, and the effective camber angle on the right side tires is approximately −0.3 degree. As a consequence of this phenomenon, the LH tires usually experience worse inboard shoulder wear than the RH tires.
When a typical tandem axle vehicle (tractor or trailer) turns, the dynamics of the vehicle favor lateral grip by the forward axle tires. As a result the pivot point of the vehicle shifts toward the forward axle tires and the rear axle tires will tend to have greater slip laterally as the vehicle negotiates a turn maneuver. For this reason, the rear tires on a tandem axle pair receive more scrub and have a faster wear rate than the tires on the forward axle. Scrub tends to arrest the development of irregular wear and thus the rear tires usually are less affected by the camber issue than are the tires on the forward axle.
So as a consequence, the tire irregular wear issue is usually worst on the inboard surface of the LF tire. Next worst is the LR tire. The RF tire comes next but is sometimes similar in severity to the LR. The most even wear usually is found on the RR tire depending upon the particular application, load, and routes normally traveled. It should be obvious that in countries such as Australia, where drivers drive on the left side of the road instead of the right side, the above would be reversed.
One mechanism of adjusting axial alignment involves a system that includes a spindle sleeve that has an outer surface about a first axis of revolution and an inner surface about a second axis of revolution at an angle to the first axis. The predetermined angle may be in a vertical direction to induce a change in camber, in a horizontal direction to induce a change in toe, or a combination thereof. The wheel can thus be set at an angle to the spindle, but doing so requires a means by which the contact surface orientation of the brake friction material can be properly matched to the friction surface of the brake disc or drum. Since the brake shoe or the brake caliper brackets are non-adjustable and rigidly attached to the axle tubing there is no way to adjust the alignment of the brake friction material to the brake contact material after the angular adjustment is made to the wheel. As such, a need exists for providing easy adjustability to the brakes so that the brake friction material can be properly aligned with the corresponding friction surface of the brake drum or brake disc.
The use of identical or similar reference numerals in different figures denotes identical or similar features.